This invention relates to a flat cathode ray display tube comprising an envelope including a substantially flat, transparent, faceplate carrying a phosphor screen, means for producing an electron beam and directing the beam parallel to the faceplate through a first region towards a reversing lens which turns the beam so that it travels in substantially the opposite direction parallel to the faceplate through a second region, first deflection means adjacent the output of the electron beam producing means for deflecting the beam substantially in a plane parallel to the faceplate and second deflection means in the second region for deflecting the electron beam toward the screen.
A flat cathode ray display tube of this kind is described in British Patent Specification 2101396. In this tube, the envelope consists of a shallow, generally rectangular, metal can with a flat glass faceplate, constituting the display window, mounted on the can. An electron gun in the rear region of the envelope produces a low energy electron beam which is deflected to effect line scanning by an adjacent electrostatic deflection arrangement before passing to the reversing lens. After having been reversed in direction through 180 degrees, the beam undergoes field scanning by means of a plurality of selectively energised, electrodes arranged in a plane parallel with the faceplate in the front region of the envelope and is deflected thereby towards a phosphor screen carried on the faceplate onto the input side of a channel electron multiplier disposed parallel to, but spaced from, the screen. Thus, the line and field scanned beam provides a raster scanning electron input to the electron multiplier. Having undergone current multiplication within the electron multiplier, the electron beam is accelerated onto the phosphor screen by means of a high voltage field established between a backing electrode on the screen and the output side of the electron multiplier to produce a raster-scanned display picture.
An advantage in using an electron multiplier in this manner is that the multiplier in effect separates the scanning function of the electron beam from the light-generating process. The electron beam, prior to reaching the multiplier, need only be of low energy so that the beam forming and raster scanning section of the tube operates at low voltage and current compared with the high voltage, higher current screen output section. The term "low energy" used herein is intended to signify an electron beam of less than 2.5 KeV and typically several hundred electron volts. For example, a low voltage, low current beam having an acceleration voltage of around 400 V may be used. The electron multiplier amplifies the beam current and the amplified current beam, is accelerated across a short gap to the screen to produce the power necessary to generate the light output. The low energy and low current electron beam used in the beam forming and raster scanning section of the tube can easily be deflected through large angles without undue enlargement of the spot. This enables the kind of folded electron optical system described to be employed with the result that a comparatively compact, and shallow, display tube is obtained.
As a result, however, of the use of a low energy electron beam in the beam forming and raster scanning section of the tube and the long trajectory of the beam in that section, the tube is more sensitive to ambient magnetic fields, for example the Earth's magnetic field, than a conventional display tube using a high voltage beam. The low energy beam is more easily disturbed and ambient magnetic fields penetrating the first region of the tube can influence the direction and position of the beam before it reaches the reversing lens producing a deviation from the intended path of the beam through the reversing lens. If it fails to enter the reversing lens within the lens' acceptance window a total loss of picture can result.
The metal can of the tube's envelope affords some magnetic shielding. In addition, an external magnetic shield comprising a box of mumetal material can be fitted around the tube's envelope, to reduce the effects of extraneous magnetic fields. However, the use of a shielding box is expensive and adds to the bulkiness and weight of the tube, causing further complications, and is not always practical.